Given the poor near-term prospects for an HIV-1 transmission-blocking vaccine and the failure of condoms as a practical mechanism for controlling sexual transmission of HIV-1, there is urgent need for an effective and acceptable anti-HIV-1 microbicide. Two features, which should dramatically enhance microbicide acceptability would be complete transparency to the user and the absence of a requirement for immediate pre-coital application. Although the relative roles of cell-free and cell-associated virus in human sexual transmission are unknown, we have found using in vivo SCID mouse and in vitro transwell models of HIV-1 sexual transmission that cell-associated transmission of HIV-1 is highly efficient and cell-free virus is poorly transmitted. These results differ from those reported in the macaque model system, which is likely accounted for by the fact that all of the reported macaque studies make no effort to adjust the environment to the neutral pH at which transmission occurs in the human setting. In our model systems, antibody to the cell adhesion molecule ICAM-1 is remarkably effective in blocking cell-associated transmission. Preliminary data suggest that the antibody is acting by a mechanism that involves signal transduction rather than by simply blocking the docking function of ICAM-1. We are now hypothesizing that lactobacilli expressing single chain antibody fragments (scFv or scAb) can be used as a delivery mechanism for providing a sustained, totally transparent,and, in the case of heterosexual transmission, woman-controlled method for blocking cell-associated virus transmission. To evaluate this hypothesis we have established a collaboration with the leading laboratory in the world studying the expression of antibody fragments by lactobacilli. In collaboration with this laboratory, located in the Netherlands, we seek to 1) Examine differential effects of engagement of ICAM-1 in a cross-linking or a non-cross-linking manner on resistance of endothelial monolayers to transmigration of infected cells 2)Engineer lactobacilli to express cell-wall bound or secreted monovalent ordivalent scFv or scAb and evaluate the in vitro efficacy of the secreted products in the transwell assay 3) Evaluate in the SCID mouse model the colonization characteristics and protective efficacy of engineered lactobacilli used to colonize the mouse vagina. It is anticipated that these studies will provide the proof of principle necessary to initiate clinical studies using this approach.